The semiconductor integrated circuit (IC) industry has experienced exponential growth. Technological advances in IC materials and design have produced generations of ICs where each generation has smaller and more complex circuits than the previous generation. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process is the result of various process changes and improvements, including more precise lithography. Such scaling down has also increased the complexity of processing and manufacturing ICs and, for these advances to be realized, similar developments in IC processing and manufacturing are needed.
With small feature sizes in advanced technology nodes, lithography patterning faces more challenges. For example, overlay error needs to be much smaller since feature size is reduced. On the other hand, lithography technology uses a radiation beam of high energy photons, such as deep ultraviolet (DUV) or extreme ultraviolet (EUV), since high energy photons have short wavelength and high resolution that enable formation of small size features.
The method for overlay control and monitoring usually includes selecting limited number of sample wafers by an experienced user and measuring only the selected sample wafers. The sampling locations and quantities on each selected wafers are also decided by the user. For example, 17 dies out of 62 dies on a wafer, and 12 points out of each selected die are decided by the user's experience for performing the overlay error measurement. This method relies on human experience, thus sometimes results in poor measurement coverage, such as full measurement in some fields and no measurement in other fields. This method may also introduce redundant measurements in some regions of the wafer which result in inefficient and ineffective processes and low throughput. Therefore there is a need for an optimized sampling strategy for overlay control and monitoring.